1. Field of the Invention
This invention relates to electrical discharge systems for ring laser gyroscopes, and more particularly, internal anode systems for ring laser gyroscopes.
2. Description of Related Art
Over the past twenty five years the gaseous medium planar ring laser gyroscope has been developed and evolved as a reliable and relatively environmentally insensitive inertial rotation sensor. Planar ring laser gyroscopes of both triangular and square geometries have been used in inertial navigation systems and flight control systems regularly in both commercial and military aircraft. The primary advantage of the ring laser gyroscope over the spinning-wheel mechanical gyroscope is its ability to be configured into a truly strapdown system. This provides a system that not only has a much larger dynamic range than the mechanical equivalent but also one free of mechanical bearings, greatly enhancing its ability to withstand relatively large mechanical shock without permanent degradation of its performance. Because of this and other features the expected mean time between failures of most ring laser gyroscope inertial navigation systems is several times longer than the mechanical gyroscopes they replace.
Components affecting the operating life of a DC discharge ring laser gyroscope include the electrodes, such as the cathode and anodes and the method for sealing such electrodes to the ring laser gyroscope body. Prior art designs are exemplified in the following U.S. Pat. Nos.: 3,390,606, issued Jul. 2, 1968 to Podgorski; 4,273,282, issued Jun. 16, 1981 to Norvell et. al.; 4,392,229, issued Jul. 5, 1983 to Hostetler; 4,432,660 issued Feb. 21, 1984 to Norvell et.al.; and, 4,837,774 to Jabr et. al. The Norvell '229 and '660 patents are directed to better sealing techniques for external electrode discharge laser gyroscopes, while Podgorski '282 shows the use of an anode positioned within the laser cavity bore, but without particular emphasis or description. In particular Podgorski's patent discusses anodes which are "positioned within the triangular cavity" but does not describe how this is accomplished or how electrical contact is made to them through the glass frame. His drawings indicate that two electrical feedthroughs are required which would incorporate two extra seals on the triangular frame. Podgorski's discharge path fills a large fraction of the lasing-light path in order to provide the necessary gain for lasing action and as such requires a larger power consumption than would be necessary if the discharge path could be shortened.
FIG. is a PRIOR ART embodiment similar to the design described regarding FIG. 1 of U.S. Pat. No. 4,392,229 (heretofore cited). The ring laser gyroscope is shown in plan view in FIG. 1. A Ring Laser Gyroscope frame 10, made of a dimensionally stable material such as quartz or Zerodur.TM., is shown generally rectangularly shaped to carry a ring laser cavity 12a through 12d. (It should be note that although the cavity 12a-12d is shown to be rectangular, it may be triangular or any other suitable polygonal shape.) The cavity 12a through 12d is filled with a laser gas such as, for example, a mixture of helium and isotopes of neon. The laser cavity 12 is shown as a planar cavity (but need not be restricted to planar configuration for all varieties of ring laser gyroscopes). Mirrors 14, 16, 18, and 20 are positioned at the vertices of the rectangular cavity 12a through 12d to create a resonant cavity. The gyroscope frame 10 defines mirror wells 14a, 16a, 18a and 20a to receive and house the mirrors 14, 16, 18, and 20. The mirrors 14, 16, 18, and 20 are angled to reflect counterpropagating laser light around the laser cavity path 12a-12d. One or more mirrors, such as 14 and 20, may be partly transmissive so that photosensors (not shown) affixed to the outer surface of the mirror substrate may sense counterpropagating light as a resonant beat signal which can be detected by appropriate electronics, yielding both rotation rate and direction or sense. The gas laser, in order to lase, requires the flow of a high voltage electrical discharge from the cathode 22 to each of the external anodes 24 and 26. Filled with a helium-neon laser gas, the active gain medium region 28 exhibits a glow discharge of orange light which, as indicated by the shaded cavity portions, extends from each of the anodes 24 and 26 to the cathode 22. The anodes 24 and 26 and the cathode 22 are hermetically sealed to the gyroscope frame 10; and, the external anodes 24 and 26 are positioned symmetrically about the cathode 22 to ensure diminished Langmuir gas flow effects. Both the cathode 22 and the anodes 24 and 26 are metallic (such as aluminum for the cathode and copper for the anodes), and the interior of the cathode 22 is preferably covered with a coating of an electron-emitting oxide (such as aluminum oxide). It can be seen that the active gain medium region 28 may extend any where from 1/3 to 1/2 of the entire length of the gyroscope cavity 12a through 12d. The PRIOR ART design contemplates an active gain medium region 28 path designed to avoid accidental backlighting between the anodes. Prior Art DC-discharge ring laser gyroscopes use anodes 24 and 26 that are mounted at separate access tubes in the gyroscope frame 10. This technique operates successfully but requires extra seals and a portion of the discharge is not used for gain.